How to remove all elements from a dictionary?

Question

I use the following code to remove all elements from a dictionary:

internal static void RemoveAllSourceFiles()
        {
            foreach (byte key in taggings.Keys)
            {
                taggings.Remove(key);
            }
        }

But unfortunately this isn't working because an InvalidOperationException is thrown. I know this is because the collection is modified while iterating over it, but how can I change that?

Note: While in this specific case `Clear` makes much more sense, in the general case you can solve this problem by iterating over a copy of the list of keys, e.g. by using `foreach (byte key in taggings.Keys.ToList())`. — Brian, Mar 09 '11 at 14:51
Thanks, this helped me. The `Clear` approach does not work in my case, because I need some events to be triggered on removal of the items. — waldgeist, Apr 20 '19 at 20:49

score 37 · Accepted Answer · answered Mar 09 '11 at 07:26

37

A much simpler (and much more efficient) approach:

taggings.Clear();

and yes, the error is because changing the data deliberately breaks iterators.

answered Mar 09 '11 at 07:26

Marc Gravell

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score 6 · Answer 2 · edited May 23 '17 at 12:02

6

Try using the Clear method instead.

internal static void RemoveAllSourceFiles()
        {
           taggings.Clear();
        }

Update: And as Marc pointed out, you cannot continue iterating over a collection while you modify it because the iterator is irrecoverably invalidated. Please read the answer to this SO question for details.

Why does enumerating through a collection throw an exception but looping through its items does not

edited May 23 '17 at 12:02

Community

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answered Mar 09 '11 at 07:26

Unmesh Kondolikar

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more accurately, you can't *continue* iterating it after modifying it; subtle difference, maybe... but you very much *can* modify the collection. – Marc Gravell Mar 09 '11 at 07:27
2

Thank you Marc for pointing it out. Yes, subtle but important difference. – Unmesh Kondolikar Mar 09 '11 at 07:32

score 4 · Answer 3 · answered Mar 09 '11 at 07:27

4

Dictionary.Clear?

answered Mar 09 '11 at 07:27

Cameron MacFarland

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score 2 · Answer 4 · answered Mar 09 '11 at 07:27

2

To do what you want to do you are going to need to iterate through the keys in reverse, that way you do not modify the array in the order it is trying to return to you.

Either that or use .Clear()

answered Mar 09 '11 at 07:27

Dested

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Eilistraee · Answer 5 · 2011-03-09T12:20:00.167

As said, the .NET default enumerator doesn't support collection changes while enumerating. In your case use Clear.

If you want better control over deletion, use linq:

var deletionList = (from tag in taggings where <where clause> select tag.Key).ToArray();
foreach(var key in deletionList)
{
   taggings.Remove(key);
}

The ToArray() extension method will enumerate the LINQ query, and instantiate an array storing the results. This array can be safely enumerated later to delete the contained items in the source dictionary.

score 0 · Answer 6 · edited May 23 '19 at 11:04

0

I know this is an old question, but for any who's looking for an answer, here some options:

To list() and then use the .remove property.
Make it null/nothing and reinit it?
Forloop in stead of foreach?

while Dict.Count()
  clDictObject oBj = Dict.Keys.ElementAt(dict.Count -1);
  Dict.Remove(oBj)
end while

edited May 23 '19 at 11:04

double-beep

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answered May 23 '19 at 08:43

John Schellingerhout

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How to remove all elements from a dictionary?

6 Answers6